Powder spray systems are well known in the art and representative systems are disclosed, for example, in U.S. Pat. Nos. 4,378,728; 4,409,009 and 4,590,884, all assigned to the assignee of this invention. As disclosed in these patents, the practice of powder coating involves the deposition of a powdered resin onto a substrate and thereafter heating the substrate and powder so that the powder melts and when subsequently cooled forms a solid continuous coating on the substrate. In many instances, an electrostatic charge is applied to the sprayed powder to increase the quantity of powder which attaches to the substrate and to assist in retaining the powder on the substrate.
Powder deposition is usually performed in a spray booth, i.e., a controlled area wherein any oversprayed powder which is not deposited on the substrate can be collected. Conventionally, the containment of oversprayed powder in the spray booth is aided by an exhaust system which creates a negative pressure within the spray booth and draws the powder entrained in a stream of air out of the spray booth into a powder recovery unit. In the powder recovery unit, the particulate powder material is separated from the air by a filter media, collected in a hopper and then usually returned to the powder source.
Many powder spray booths, including those disclosed in the patents listed above, employ cartridge filters as the filtering media within the powder collection chamber. Cartridge filters are conventionally cylindrical in shape having a filtering wall with a pleated outer surface and a hollow interior. These cartridge filters are mounted within a powder recovery chamber formed in the powder recovery unit at entrance openings to a filtered or clean air chamber therein. A blower carried in the powder recovery unit creates a negative pressure within the powder recovery chamber which draws air-entrained, oversprayed powder from the spray booth into contact with the cartridge filters in the powder collection chamber. The pleated outer surface of the cartridge filters is effective to collect most of the oversprayed powder withdrawn from the spray booth so that the air passing through the cartridge filters to the clean air chamber is filtered and substantially free of particulate powder material.
Periodically, the wall of the cartridge filters becomes so filled with collected particulate powder material that it must be cleaned in order to continue to effectively filter the particles from the air entering the collection chamber. Cleaning of the cartridge filters in prior art powder spray booths has been accomplished by either vibrating the particles off the wall of the filter or directing a reverse air flow through the filter, i.e., from the interior to the exterior, which dislodges the particles from the filter wall so that they fall into a collection hopper located beneath the filters.
As disclosed, for example, in U.S. Pat. No. 4,409,009, the reverse flow or jet of air employed to clean the cartridge filters is often directed through a venturi throat mounted within the clean air chamber at the openings over which the cartridge filters are mounted. These venturi throats accelerate the jet or blast of air into the interior of the cartridge filter which forms a shock wave therein to reverse the flow of air through the filter wall so that the particulate powder material is dislodged therefrom. After the short blast or jet of cleaning air is terminated, the flow of air in the opposite direction through the cartridge filters is resumed.
One problem with air blast cleaning systems for cartridge filters is that a relatively high pressure blast or jet of air, i.e., on the order of 90 psi, is required in order to effectively dislodge collected particulate powder material from the filter wall. The production of high pressure compressed air within a manufacturing facility requires equipment which is relatively expensive to purchase and operate. Even the periodic bursts or jets of high pressure air needed to clan the cartridge filters involves a substantial volume of compressed air which is expensive to produce.
Another problem with prior art air jet systems for cleaning cartridge filters is that venturi throats are often used to increase the speed of the air jet and ensure effective cleaning of the cartridge filter. As mentioned above, these venturi throats are commonly positioned in the clean air chamber above the cartridge filters, and, in turn, the air jet nozzle or other means for discharging the air jet is positioned above the venturi throat. This construction adds several inches to the overall height of the powder recovery unit which is unacceptable given the space requirements of some facilities.